Style fix to comply with dartfmt

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[f3ath]

I signed it!

[googlebot]

CLAs look good, thanks!

[natebosch]

Looks like someone else also had the same idea - the modern syntax is used today https://github.com/dart-lang/tuple/blob/366318b7cb1d7c79dc00a6852684ff7af518a612/lib/tuple.dart#L61